1. Field of the Invention
The present invention relates to a dispersion-compensating optical fiber for compensating for positive chromatic dispersions of typical single-mode optical fibers in a 1.5-xcexcm wavelength band, and an optical transmission system in which this dispersion-compensating optical fiber is employed as an optical transmission line.
2. Related Background Art
Since optical fibers mainly composed of silica glass yield the lowest transmission loss with respect to light in a 1.55-xcexcm wavelength band, optical transmission systems employing optical fibers as their optical transmission lines utilize light signals in the 1.55-xcexcm wavelength band. Typical single-mode optical fibers having their zero-dispersion wavelength in a 1.3-xcexcm wavelength band, on the other hand, have positive chromatic dispersions in the 1.5-xcexcm wavelength band. As the chromatic dispersion is greater, the waveform of each light signal is more likely to deteriorate, and the wave form of each light signal further deteriorates upon interactions between the chromatic dispersion and nonlinear optical effects. Therefore, in order to compensate for the positive chromatic dispersions of typical single-mode optical fibers in the 1.5-xcexcm wavelength band, i.e., in order to reduce the chromatic dispersion of the whole optical transmission line, dispersion-compensating optical fibers having a negative chromatic dispersion in the 1.5-xcexcm wavelength band and the above-mentioned typical single-mode optical fibers are combined together.
For example, document 1xe2x80x94T. Kashiwada, et al., xe2x80x9cBroadband dispersion compensating module considering its attenuation spectrum behavior for WDM system,xe2x80x9d OFC""99 Technical Digest, WM12 (1999)xe2x80x94and document 2xe2x80x94L. Grunder-Nielsen, et al., xe2x80x9cDesign and manufacture of dispersion compensating fibre for simultaneous compensation of dispersion and dispersion slope,xe2x80x9d OFC""99 Technical Digest, WM13 (1999)xe2x80x94disclose dispersion-compensating optical fibers such as one having the refractive index profile 450 shown in FIG. 1. The conventional dispersion-compensating optical fiber shown in the drawing comprises a core extending along an optical axis and having a refractive index n1; a depressed region provided so as to surround this core and having a refractive index n2 ( less than n1); and a cladding provided so as to surround the depressed region and having a refractive index n3 ( greater than n2,  less than n1). In the refractive index profile 450 shown in FIG. 1, areas 451, 452, and 453 indicate refractive indices of the core, depressed region, and cladding, respectively.
The inventors have studied the above-mentioned conventional dispersion-compensating optical fibers and, as a result, have found problems as follows. Namely, the conventional dispersion-compensating optical fibers having a refractive index profile such as the one shown in FIG. 1 have a small effective area while confining a large amount of light into the core, whereby nonlinear optical effects such as self phase modulation are likely to occur therein. Therefore, optical transmission systems employing these dispersion-compensating optical fibers as their optical transmission lines cannot transmit light signals having a high power, and thus must shorten the spacing between repeater stations including optical amplifiers for optically amplifying light signals, which inevitably increases the number of repeater stations needed.
Also, since the conventional dispersion-compensating optical fibers having a refractive index profile such as the one shown in FIG. 1 have a shorter cutoff wavelength for fundamental-mode, cutoffs caused by disturbances such as microbend and macrobend have been affecting light on the shorter wavelength side, thereby increasing the transmission loss in the signal wavelength band. Therefore, also from this point, the distance between repeaters including optical amplifiers and the like has become shorter in the optical transmission systems employing the conventional dispersion-compensating optical fibers as their optical transmission lines, thus necessitating a number of repeaters to be provided (lowering performance per cost).
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide a dispersion-compensating optical fiber capable of transmitting, with a low loss, light signals having a high power; and an optical transmission system employing the same.
The dispersion-compensating optical fiber according to the present invention is insured its single mode at a wavelength of 1.55 xcexcm and comprises, at least, a core region having a first core extending along a predetermined axis and a second core provided on the outer periphery of the first core, and a cladding provided on the outer periphery of the core region. The first core has a refractive index n1 and an outside diameter 2a. The second core has a refractive index n2 higher than that of the first core and an outside diameter 2b. The cladding has a refractive index n3 lower than that of the second core.
In particular, the dispersion-compensating optical fiber according to the present invention has a chromatic dispersion of xe2x88x9210 ps/nm/km or less at the wavelength of 1.55 xcexcm, whereas the ratio 2a/2b of the outside diameter 2a of the first core with respect to the outside diameter 2b of the second core is 0.05 or more. Due to the foregoing configuration, this dispersion-compensating optical fiber not only compensates for chromatic dispersions of typical single-mode optical fibers, but also can suppress the generation of nonlinear optical effects more effectively by yielding a larger effective area.
The dispersion-compensating optical fiber according to the present invention may comprise a depressed region provided between the second core and the cladding. Here, the depressed region has a refractive index n4 lower than that of each of the second core and the cladding. Due to this configuration, the dispersion-compensating optical fiber has a longer cutoff wavelength for fundamental-mode, and yields a negative dispersion slope at the wavelength of 1.55 xcexcm. Since the dispersion slope at the wavelength of 1.55 xcexcm is negative as such, the dispersion-compensating optical fiber can compensate for both chromatic dispersion and dispersion slope of typical single-mode optical fibers. The dispersion-compensating optical fiber may further comprise an intermediate region having a refractive index n5 ( greater than n3,  less than n2) provided between the depressed region and the cladding region.
In the dispersion-compensating optical fiber according to the present invention, the ratio 2a/2b of the outside diameter 2a of the first core with respect to the outside diameter 2b of the second core is 0.6 or less. Setting the outside diameter ratio between the first and second cores as such can effectively suppress the increase of loss caused by bending. Specifically, the increase of loses in the dispersion-compensating optical fiber with respect to light having a wavelength of 1.55 xcexcm when wound by one turn about a mandrel having a diameter of 32 mm is 0.5 dB or less. As a consequence, even if disturbances such as microbend and macrobend occur upon cabling, the increase of transmission loss in the signal wavelength band can be suppressed effectively.
On the other hand, the optical transmission system according to the present invention comprises, as a WDM (Wavelength Division Multiplexing).optical transmission line, a dispersion-compensating optical fiber having a structure such as one mentioned above; and another optical fiber, optically connected to the dispersion-compensating optical fiber, having a positive chromatic dispersion at the wavelength of 1.55 xcexcm. Due to such a configuration, both chromatic dispersion and dispersion slope of the whole optical transmission system are reduced in a wavelength band in use, and the signal distortion of each signal wavelength caused by chromatic dispersion and nonlinear optical effects is effectively suppressed. Here, the dispersion slope is given by the differential coefficient of the chromatic dispersion.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.